The present invention relates to a procedure and an apparatus for braking a squirrel-cage elevator motor fed by a frequency converter in fault situations.
In certain countries, the elevator regulations stipulate that gearless elevators should be capable of so-called "dynamic braking", which means that it must be possible even in exceptional situations to decelerate the elevator by means of its motor. For instance, in the event of failure of the brake arms the motor must be able to reduce the speed of the elevator car.
In the d.c. (direct current) drives currently used, this problem has been solved by connecting a resistor to the armature winding in failure situations, thus enabling the motor to brake by acting as a generator. The regulations generally assume that the main circuit is inoperative or inaccessible, thereby preventing normal motor action. In the case of an elevator equipped with a squirrel-cage motor, a corresponding procedure cannot be applied because without its main circuit the motor will not be magnetized, and therefore cannot generate a torque.